The use of Long Term Evolution (LTE) wireless communication in unlicensed bands is being standardized as part of the Third Generation Partnership Project (3GPP) Release 13 as Licensed Assisted Access LTE (LAA-LTE). As part of LAA-LTE, it has been agreed that LTE access to the unlicensed band, i.e., unlicensed spectrum, will use support through Carrier Aggregation (CA) with a licensed band LTE carrier. The LTE primary component carrier in the licensed band is aggregated through CA with an LTE secondary component carrier in the unlicensed band. The primary carrier contains scheduling and control channels for scheduling user plane traffic across both the licensed band primary and unlicensed band secondary carrier. Thus, the secondary carrier in the unlicensed band will typically carry user plane traffic, and may be shut off when no traffic is required over the unlicensed band carrier in order to minimize interference to other systems operating in the same unlicensed band, e.g., Wi-Fi® equipment.
For LAA-LTE User Equipment (UE) to synchronize to the secondary carrier in the unlicensed band, an LAA-LTE access point, e.g., evolved node B (eNB), transmits a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), and a Common Reference Signal (CRS) at periodic intervals. Typically, the PSS and SSS are sent every five subframes (i.e., every 5 milliseconds), and the CRS is sent every subframe. LTE UEs rely on the PSS to initially time synchronize at the subframe level. Once synchronized at the subframe level, the UE can read the SSS to obtain the Physical Cell Identifier (PCI) for the LTE carrier. One the UE reads the SSS, it knows the location of the CRS for doing channel estimation. In normal LTE operation, the PSS, SSS, and CRS are transmitted continuously, even on the secondary component carrier when there is no user plane data to send.
UE cell search and channel estimation are important components of a wireless network. These processes are typically aided by control signals, such as PSS/SSS and CRS, which are transmitted periodically. However, due to the uncertainty of channel availability in the unlicensed band, the exact transmission timing of these signals cannot be guaranteed, thereby degrading the performance of the entire process.